1. Field of the Invention
This invention relates to a discharge pumped laser device used for light sources for machining works, projection light, etc. and more particularly to discharge electrodes in the laser device which minimize the fluctuation of the beam width of the laser beam due to the abrasion of the electrodes.
2. Description of the Related Art
A discharge pumped laser device is used for machining of works such as marking, cutting and welding, and also used as a light source for optical lithography to transcribe circuit patterns for large scale integrated circuits.
For machining works, carbon dioxide lasers, excimer lasers, etc., are used. For optical photolithography, a reduction projection process is typically used. In the reduction projection process, light irradiated on and passed through a reticle pattern is projected through a reduction projection aligner onto a photo-sensitive material deposited on a semiconductor substrate to thereby form a circuit pattern. The resolution of the projected image is inverse proportional to the wavelength of light; therefore, the wavelength of light has been shifted from a visible region to an ultraviolet region in order to improve the resolution.
Conventionally, g-line (having a wavelength of 463 nm) and i-line (having a wavelength of 365 nm), which are produced by a high pressure mercury lamp, are used as an ultraviolet light source. However, for the production of a 64 megabyte memory, the line width is 0.25 m or less at the minimum pattern. Therefore, a light source having a wavelength shorter than those of the i-line has been expected.
A deed ultraviolet (deep-UV) laser light source is now considered promising to break the technological limit for shorter wavelength. Especially, the excimer laser produces high output with high efficiency. It provides strong oscillation at short wavelengths in a certain composition of the medium gas such as KrF (wavelength of 246 nm) and ArF (wavelength of 193 nm).
However, the selection of glass and crystalline materials which constitute the reduction projection lens system is greatly restricted in the deep-UV region. Therefore, the correction of the chromatic aberration is difficult to perform in the reduction projection lens system which uses a mercury lamp. In place of correcting the chromatic aberration of the lens system, a wavelength selection device such as an etalon is disposed in the laser cavity (resonator) to thereby reduce the spectrum width of the output light to such an extent that the chromatic aberration is negligible. According to this method, the output having a spectrum width of several nanometers in the natural oscillation can be reduced to a narrow band of several picometers. To reduce the bandwidth, it has been proposed the methods in which one or more Fabry-Perot etalons including a pair of parallel reflective films are provided the cavity or a diffraction grating is used as a total reflection mirror.
The former method uses a plurality of etalons which are superposed to each other. This method can reduce the bandwidth relatively easily. However, since the optical intensity between the reflective films is very high, the durability of the reflective films and a drift due to the loss become problems when high output is to be produced.
The latter method using a diffraction grating can achieve wavelength selection by a single reflection on the diffraction grating surface so that the optical intensity is lower than that of the former method. As a result, the load on the reflective surface is small. Further, since the reflection takes place only one time, a loss at the reflective surface is low and high output is obtained. Because of these advantages in the latter method, a narrow band excimer laser device widely employs a diffraction grating as a light source for optical lithography.
Referring to FIG. 23(a), a narrow band excimer laser is of a discharge pumped type in which laser oscillation is performed by discharging and pumping a laser gas such as KrF filled in a laser chamber 1 through a pair of electrodes 6, 7 disposed above and below the laser chamber along the longitudinal axis of the chamber. A discharge pumped region 11 is shown enclosed by the broken lines. As discharge advances, the opposing surfaces 6a, 7a of the electrodes 6, 7 are abraded and the discharge width WA changes. Along with the change in discharge width, the beam width of the output laser light 4 changes, which is a problem in terms of stabilization of the beam width.
When this laser is used for machining works, due to the change in the beam width, the transverse mode is deteriorated, the beam condensing performance is changed, and the output is changed. This causes a practical problem. Especially, when this laser is used as a light source for optical photolithography, a change in the beam width would cause the following undesirable problems in the practical use of the narrow band excimer laser.
To reduce the bandwidth, the diffraction grating 5 (FIG. 23(a)) is used as a wavelength selective device and high order diffraction is used in the diffraction grating, so that angular dispersion at the operating point becomes large and the divergent angle of the laser beam directly influences the spectrum width. When the divergent angle is large, the Spectrum width would increase. Therefore, when the discharge width, that is the gain region, changes, the divergent angle of the laser beam changes, so that the spectrum width greatly changes. In order to avoid this change, an aperture 8 (FIG. 23(a)) is conventionally provided to stabilize the gain width. In a so-called Chang type electrodes used conventionally, the discharge width would greatly increase inevitably in the abraded electrodes when the output is increased. As a result, it becomes difficult to control the output due to the restriction on the gain by the aperture 8.
The mechanism of enlarging the discharge width will now be described. Townsent's theory is known as a macroscopic phenomenological theory which explains discharge, and is useful for understanding discharge phenomena. In this theory, a gas including a halogen used in the excimer laser is called a negative gas. Electrons produced by the collision and ionization of the electrons (ionization coefficient: .alpha.) in the discharge process by a large electron affinity of halogen are captured (electron attachment coefficient: .eta.) to reduce an apparent ionization coefficient (.alpha.-.eta.) to thereby facilitate condensation of the discharge. The relationship between these coefficients and electric field strength E is shown in FIG. 24. in which a reference character P denotes a normalization coefficient.
As seen from FIG. 24, the ionization coefficient is greatly dependent on the electric field strength E while the electron attachment coefficient .eta. is not substantially dependent on the electric field strength E, so that the apparent ionization coefficient (.alpha.-.eta.) rapidly increases in excess of a certain electric field strength and greatly depends on the electric field strength E. Thus, the parameter (.alpha.-.eta.) which drives the discharge changes in accordance with the electric field strength distribution on the surface of the electrodes, so that the discharge width is greatly influenced by the electric field strength distribution on the electrode surface. In order to ensure the discharge width, a uniform large electric field strength region transverse to the electrodes of the discharge pumped type laser device is required to be provided as its electrode shade. Conventionally, the shades of Chang and modified Chang electrodes are used on the basis of the analysis of the electric field under ideal conditions.
FIG. 25(a) shows a potential distribution derived from the electric field calculation in the case of a modified Chang type electrodes, in which L1 . . . denotes equipotential lines. It will be seen from FIG. 25(a) that the potential distribution between a cathode (upper electrode) 6 at high negative potential and an anode (lower electrode) 7 is greatly bent by a metal plate on which the anode 7 is placed, a current return lead 10 and an insulating member 9. In the actual configuration, the existence of such leads and isolation causes the electric field to deviate from an ideal Chang type electric field.
FIG. 25(b) shows equifield strength line L2 . . . in the vicinity of the cathode electrode 6 while FIG. 25(c) shows changes in the electric field strength along the surface of the cathode electrode 6. As seen from FIG. 25(b), the electric field strength on the surface of the electrode 6 does not so often intersect the equifield strength lines in the region ranging from the electrode central point A to about 1/3 of the right half electrode width and a uniform electric field is formed in that region.
As shown by a line L3 in FIG. 25(c), the electric field intensity initially rises very slowly from the electrode central point A toward the right-hand electrode end point B and then rises more rapidly closer to the end point. FIG. 25(d) shows an equifield strength line L4 . . . in the vicinity of the anode electrode 7. FIG. 25(e) shows changes in the electric field strength along the electrode surface. The electric field strength at the center A is denoted by E.sub.0. As shown by these figures, the electric field strength is uniform in the first rightward section of about 4 mm starting from the electrode central point A. It then becomes slowly lower toward the right-hand end B and becomes rapidly lower after a distance of about 12 mm (see lines L4, L5). This is a trend reverse to that in the case of the cathode 6. Discharge occurs at the central portion of the electrode. The factor of restricting the discharge width is considered to be on the anode 7 side where its central portion is at high electric field strength.
After 1.times.10.sup.8 shots, the shape of the abraded electrode central portion was measured and the electric field strength distribution in the abraded electrode central portion was calculated by using finite element method on the basis of the measured shape data.
FIGS. 26(a) and 26(b) show the result of this simulation in which the axis of abscissa expresses a rightward distance from the center A and the ordinate axis shows the ratio of a change .DELTA.E in the electric field strength E to electric field strength E.sub.0 at the central point A (%). FIG. 26(a) concerns the cathode 6 while FIG. 26(b) concerns the anode 7. The white dot shows the state of new electrode while the black dot shows the state after 1.times.10.sup.8 shots.
As seen from these figures, abrasion has increased after 1.times.10.sup.8 shots, so that a uniform electric field has extended in the vicinity of the central point compared with that of new electrode. The width of this uniform electric field portion corresponds to the observed beam width. In this respect, it is considered that an electric current is concentrated at the uniform electric field portion which is a strong electric field and abrasion has increased to thereby form a wide uniform electric field portion.
As described above, when the operation of the laser device starts, the conventional electrode is abraded as time passes to thereby increase the area of the uniform electric field portion and to cause the fluctuation of the beam width of the output laser beam.